This invention relates to a polyolefin composition for electrical insulation which is far less vulnerable to the deterioration of insulation that occurs as dendroid voids (referred to as "water trees" hereunder) in polyolefin or crosslinked polyolefin employed as an electrical insulation, particularly for high-tension power cables which are operated under water.
It has been known that water trees grow from voids and foreign matters, such as metals, in electrical insulation made of polyolefin or crosslinked polyolefin composition when the insulation is subjected to a high voltage under water. In spite of continued efforts for improvement of the manufacturing process, the complete elimination of foreign matters and voids is still difficult to achieve.
As a means for preventing water trees, incorporation of a small quantity of strong electrolyte into polyolefin was proposed in Japanese Patent Publication No. 24938/1977. On the other hand, there is a strong theory that the presence of metal ions is a cause of water trees.
The idea of adding a voltage stabilizer or alcohol to polyolefin was proposed several times as a common countermeasure for water trees. For example, Japanese Patent Laid-open No. 16746/1978 teaches that the addition of monohydric higher alcohol having 6 to 24 carbon atoms is effective. However, such low-molecular weight alcohol is mobile through polyolefin molecules and blooms easily to the surface of the insulation as the power cable gets hot, with the result being that the bloomed alcohol evaporates and the effect of the alcohol is soon lost. Furthermore, Japanese Patents Laid-open Nos. 119937/1974, 40547/1977, 40553/1977, and 40554/1977 describe supplementarily the incorporation of a polyhydric alcohol into an electric insulation which is made resistant by adding as an indispensable ingredient a voltage stabilizer, such as ferrocene, dihydronaphthalene, dialkoxynaphthalene, phosphorane, and halogenated p-nitroaniline. These patents claim that the dielectric strength is further improved by the addition of a polyhydric alcohol, but they treat the polyhydric alcohol as a mere additive similar to dispersants, surfactants, unsaturated polymers, etc. However, the significance of adding a polyhydric alcohol is barely recognized in the examples, and the polyhydric alcohols mentioned in these patents broadly vary in their molecular weight. What is given great importance in these patents is not water trees which readily occur in an insulation placed under water, but rather the conventional treeing that is attributable to high voltage alone, not associated with the presence of water.
Another countermeasure for water treeing disclosed in Japanese Patent Laid-open No. 145799/1977 covers the addition of a large quantity of inorganic filler. In this patent, the addition of inorganic filler together with diethylene glycol was practiced in some examples. However, the reason why it was added is not described. Examples without diethylene glycol were rather superior to examples with this substance. Japanese Patent Laid-open No. 3700/1978 discloses the incorporation of a water tree inhibitor. The example of this patent illustrates a composition to which polyethylene glycol distearate is added in small quantities. However, the effect of this substance as a water tree inhibitor is just as mediocre as the effect of a metal salt of stearic acid. This laid-open patent enumerates fatty acid, fatty acid metal salt, fatty acid ester, etc. in the same category. This fact suggests that fatty acid is an indispensable moiety which must have a group that can catch water readily. Such a group, however, is not identified in the specification, and, needless to say, nothing is mentioned about polyethylene glycol.
Japanese Patent Laid-open No. 60346/1979 discloses a modifier which is very effective for a crosslinked polyethylene composition. Such a modifier should be a compound of less than 24 carbons containing in the molecule both a carboxyl group or its metal salt with a high affinity for foreign matters such as metals and a double bond that forms chemical bonding with the polyolefin.
After a series of experiments on the prevention of water treeing, it was concluded that, contrary to the conventional teachings, a small quantity of high-molecular weight polyethylene glycol is all that is required, and no voltage stabilizer nor carboxylic acid or its salt is necessary. As pointed out before, the prior art teaches the use of ethylene glycol or diethylene glycol as an adjuvant for a voltage stabilizer. Other prior art discloses polyethylene glycol end-blocked by stearic acid as a water tree inhibitor. What differentiates the present invention from the prior art is that the problem can be solved simply by adding a small quantity of high-molecular weight polyethylene glycol, which is one of the cheapest compounds, to polyolefin.
One of the above-mentioned items of prior art maintains that a polyhydric alcohol is effective secondarily. In this prior art, however, polyethylene glycol is treated as the same category as monomeric ethylene glycol, propylene glycol, butane diol, etc. and alicyclic and aromatic polyhydroxyl compounds. In contrast with these teachings, the present invention discloses that only high-molecular weight polyethylene glycol is effective and polypropylene glycol is not. Accordingly, it is believed that high-molecular weight polyethylene glycol is quite different in its functional mechanism from polyhydric alcohols which have heretofore been used secondarily in combination with a voltage stabilizer and the like.